The geology of Taconic thrust sheets
and surrounding carbonates of the west central Vermont marble
belt,
north
of Rutland, Vermont
Kay Bierbrauer 1990
A thesis presented to the Faculty of the State University of New
York
at Albany in partial fulfillment of the requirements for the degree
of
Master of Science
College of Science and Mathematics, Department of Geological
Sciences
Advisor: W.S.F. Kidd

ABSTRACT
The carbonates of the Vermont Valley and the overlying rocks of
the
Taconic sequence have been generally believed to lie on the east
limb
of
a major unfaulted syncline (Middlebury Synclinorium). In this view
a
westward
dipping, north-south trending basal Taconic overthrust must be
exposed
somewhere along the eastern margin of the Taconic Range.
In contrast, this study based on detailed mapping at the north end
of the Taconic Allochthon suggests that a folded overthrust
surface is
only locally seen in fensters; more commonly the basal obduction
surface
of the Taconic Allochthon has been truncated by later
cross-cutting
thrust
faults. In the study area these newly recognized east dipping
faults
are
the "Whipple Hollow Fault" and the "Proctor Fault". These two
faults
postdate
the initial Taconic overthrust and must belong to the Champlain
thrust
system of which another fault forms the frontal thrust of the
Taconic
thrust
belt towards the west. Where exposed the basal thrust is
characterized
by a thick melange unit and isolated slivers of shelf carbonates.
The
melange
underlies wide areas which had previously been mapped as part of
the
authochthonous
shelf sequence. Other areas which were also believed to represent
these
authochthonous phyllites have been identified as characteristic
black
pelitic
Taconic lithologies. The redefined Taconic rocks also include
typical
green
and greenish gray phyllites of the Cambrian Bull Formation. It
indicates
that the Middle Ordovician shale unit which conformably to
unconformably
overlies the carbonate shelf west of the Taconic Range is not yet,
or
perhaps
only locally, exposed at the present erosion level of the Vermont
Valley.
In addition the easternmost equivalents of this unit must have
been
tectonically
removed and incorporated into the basal melange. Where a
lithological
correlation
with the Taconic stratigraphy is not possible the remaining black
phyllites
are probably Middle Ordovician in age; still these phyllites have
been
incorporated into the overlying melange and must be viewed as
allochthonous.
The rocks of the study area underwent a progressive deformation
path
which
can be subdivided into three different stages (D1 to D3). During
each
stage
a characteristic thrust system (T1 to T3) was active.
D1-deformation
describes
the stacking of the Taconic lithologies in an accretionary
imbricate
fan
thrust environment (T1). This early deformation is associated with
a
prominent
slaty cleavage (S1). Large-scale F1-folds, if present, would be
strongly
refolded and tightened so that they are not detectable. Obduction
of
the
composite T1-thrust stack onto the carbonate shelf resulted into a
small-scale
imbricated T2-marble/phyllite schuppen structure. The propagation
of
this
duplex has produced large-scale folds. F2-structures are
associated
with
a main regional crenulation cleavage (S2). Progressive shortening
during
D3-deformation culminated in foreland directed thrust faults (T3).
A
second
crenulation cleavage (S3) is related to this late thrusting event.
The
regional application of the observed thrust geometry strongly
suggests
that the Middlebury Synclinorium is unlikely to be an unfaulted
structure.
In particular this study suggests major north/south trending,
eastward
dipping late thrust faults for the entire length of the eastern
Taconic
margin and the Vermont Valley. The "higher Taconic slices" are
also
believed
to be related to this period of thrusting; this out-of sequence
imbrication
by later faults explains the existing "stacking controversy" among
Taconic
geologists.